1. Field of the Invention
The present invention relates to Field Effect Transistor (FET) microprocessors, and more particularly to a line driver circuit for reducing off-chip electrical disturbances in an FET processor by controlling the rise time and fall time of input signals.
2. Description of the Prior Art
Circuits are available in the prior art which attempt to control signal rise time. In U.S. Pat. No. 4,132,906 issued Jan. 2, 1979 to Allen, entitled CIRCUIT TO IMPROVE RISE TIME AND/OR REDUCE PARASITIC POWER SUPPLY SPIKE CURRENT IN BIPOLAR TRANSISTOR LOGIC CIRCUITS, an improvement in output rise time and a reduction in the parasitic power supply strike current is achieved in bipolar transistor logic circuits using an AC coupled feedback circuit. During the low to high transition of the output, an internal voltage is AC coupled to an amplifier which in turn provides a low impedance path at the base of a lower output drive transistor which is coming out of conduction.
In this patent, a dual (upper and lower) transistor push-pull output drive stage for a bipolar transistor logic circuit is described. Under low capacitance load conditions, a fast transient voltage occurs on the collector of the lower transistor. Thus a large amount of current is coupled into the base of the transistor while it is coming out of conduction. This current into the base of the lower output transistor will tend to turn the transistor back on so that for a period of time both the upper and lower output driver transistors are carrying current directly from the power supply through the two transistors to ground. As a result there may be current spikes drawn from the power supply which in turn waste power and introduce noise into the rest of the system.
Thus, to avoid the spike between the output transistors during turn-on, an uncontrolled feedback signal is coupled to the in-phase signal to turn off the lower transistor. This circuit only speeds up the turn-off time of the device, and actually increases the di/dt of the turn-off device.
In U.S. Pat. No. 3,436,563 issued Apr. 1, 1969 to Regitz entitled PULSE DRIVER WITH LINEAR CURRENT RISE, a pulse driver circuit is described wherein the emitter circuit of the output transistor includes a resistor and inductor in series. Two feedback transistors provide variable shunts of the input drive pulses. One, driven by the voltage across the inductor, limits the rate of increase of output current to a constant value. The second, driven by the voltage across the inductor and resistor, limits the output pulse amplitude to a constant value.
In this circuit, the obtaining of a feedback signal from an inductance requires an undesirably high voltage and results in a linear ramp (di/dt) of current which is not suitable for off-chip drivers. This circuit also only deals with turn-on, not turn-off.
In U.S. Pat. No. 3,914,623 issued Oct. 21, 1975 to Clancy entitled WAVEFORM GENERATOR INCLUDING MEANS FOR AUTOMATIC SLOPE CALIBRATION, a waveform generator such as a sawtooth wave generator is described wherein automatic slope calibration is provided. A variable resistance element is feedback-responsive and provides an input voltage which affects waveform shape. The shape of the waveform is controlled automatically by a time error signal through the use of a feedback loop which controls the input voltage.
The following patents are cited because they are related to the general area of signal slope control:
______________________________________ U.S. Pat. No. 4,029,972 Fox et al U.S. Pat. No. 3,973,145 Schmitt et al U.S. Pat. No. 3,535,555 Heimer U.S. Pat. No. 3,868,519 Green U.S. Pat. No. 3,909,633 Hall U.S. Pat. No. 4,250,410 Moench et al U.S. Pat. No. 3,906,254 Lane et al U.S. Pat. No. 4,216,393 Gillberg et al U.S. Pat. No. 4,314,166 Bismarck ______________________________________
IBM Technical Disclosure Bulletin Vol. 25, No. 3A August 1982 at page 997 by Gaudenzi et al
IBM Technical Disclosure Bulletin Vol. 25, No. 3A August 1982 at page 999 by Gaudenzi et al
IBM Technical Disclosure Bulletin Vol. 25, No. 3A August 1982 at page 1264 by Parks et al.